The use of a multiple fiber as an image-transmitting means in various fields has been entertained. A multiple fiber made of a multicomponent glass has been already put to practical use as an image-transmitting means of an industrial image-scope. However, since the multiple fiber made of a multicomponent glass has a fatal defect in that a long fiber cannot be used due to high light-transmission loss inherent in multicomponent glasses, the industrial image-scopes presently used are limited to those having a short length, in which the length of its image-transmitting means is at most from about 2 to about 5 m. On the other hand, in case of observing the inside of large-sized apparatuses, such as a nuclear reactor, an image-scope having a long image-transmitting means of 10 m. or more in length is necessary. A multiple fiber formed of silica glass optical fibers is suitable for use as such an image-transmitting means because of low light-transmission loss.
However, the multiple fiber of silica glasses is very difficult to produce when compared to the multiple fiber of multicomponent glasses. For this reason, in spite of having the advantage as mentioned above, it is the present situation that a method for the production has never been concretely proposed, to say nothing of putting it into practical use.
A conventional method of producing a multiple fiber and the reason why the production of a silica glass multiple fiber is difficult are explained below.
A multiple fiber is produced by bundling a multiplicity of glass light-conducting elements, heating the bundle to a high temperature and drawing it from its one end, whereby the cross-sectional area of respective elements is reduced and, simultaneously the elements adjacent to each other are fused together.
In general, the multicomponent glass has a low softening point of not more than 1,000.degree. C. and accordingly is easy to process Also, since its melt viscosity is low, the light-conducting elements made thereof, which are molten and drawn in a drawing step, are easily fused together.
In contrast to this, in the case of silica glasses; first, the softening point thereof is high, i.e. a temperature of not less than 1,800.degree. C., which makes a stable operation in the drawing step difficult, wherein it is necessary to heat a multiplicity of light-conducting elements uniformly to a temperature higher than the softening point. Further, the silica glasses are considerably high in melt viscosity, for instance, pure silica glass has a high viscosity on the order of 10.sup.4 poises even at a high temperature of 2,200.degree. C. at which the vaporization becomes noticeable. For this reason, the light-conducting elements of silica glasses which are drawn in the molten state are far inferior in flowability (deformability), when compared to light-conducting elements of multicomponent glasses in the molten state. Consequently, the elements are not uniformly fused to each other and a large number of bubbles are produced in the multiple fiber obtained by drawing. The bubbles result from expansion of a gas (mainly air), which has been entrapped between the light-conducting elements due to uniform fusing of the light-conducting elements together in the drawing step, at the high temperature upon drawing. Upon the formation of bubbles, the light-conducting elements around the bubbles partially sustain a large deformation, thus resulting in noticeable partial change in fiber diameter and curving. The noticeable partial change in fiber diameter and the local curving increase light scattering loss in the light-conducting element, or in individual optical fibers when the element is composed of a multiplicity of optical fibers as mentioned below. This increase of the scattering loss remarkably decreases the quantity of light that the light-conducting elements or the optical fibers are capable of conducting, consequently resulting in lowering of the image-transmitting capacity. When a multiple fiber including such light-conducting elements or optical fibers is used as an image-transmitting means, dark points are produced in the transmitted image. Thus, the occurrence of a large number of bubbles in the multiple fiber produces, in large numbers, light-conducting elements or optical fibers defective in image-transmitting capacity, consequently lowering the image-transmitting capacity of the multiple fiber itself.